"Mitt Romney's not the solution," declares an ad approved by Barack Obama. "He's the problem."

"Mitt Romney's not the solution," declares an ad approved by Barack Obama. "He's the problem." (Obama For America)

When the Olympic Games began almost 30 centuries ago in ancient Greece, rulers of city-states proclaimed an "Olympic truce," a ban on warfare to allow athletes, poets and spectators to attend without getting speared.

It would be nice to think that this year's Olympics might turn into the occasion for a domestic political truce, a summer vacation from the ferocious character attacks each side has been leveling at the other's candidate.

But it's not going to happen. Why? Because negative advertising works.

Republicans have hammered President Obama for his statement that entrepreneurs who achieved success in business "didn't build that" by themselves, charging that it was evidence that he doesn't respect hard work. Democrats have pounded Romney for his career at Bain Capital (and his not-ready-for-"American Idol" singing voice), accusing him of sending American jobs to China and Mexico.

The result? Both Obama and Romney now enjoy — if that's the right word — the lowest public esteem of any two presidential candidates since pollsters began measuring this stuff.

An NBC News/Wall Street Journal poll released June 24 found that both candidates were viewed "very negatively" by record high numbers of voters — 32% for Obama, 24% for Romney.

Obama can take some solace in the fact that more voters still view him favorably than unfavorably, 49% to 43%. Romney is in negative territory: 35% favorable, 40% unfavorable.

And Romney faces another challenge: It's not a fair fight.

Most voters already know what they think about Obama: They've been evaluating him — first as a candidate, then as president — for more than four years.

"There's almost no such thing as 'don't know' when it comes to Obama," said Douglas Rivers, a Stanford political science professor who also runs the YouGov polling firm. "There's no new information flowing in."

In the NBC/Wall Street Journal poll, in fact, exactly zero percent of voters said they don't know what they thought about Obama — although 8% said they felt "neutral," which comes close.

So when Republican groups spend millions on advertising calling Obama a failure, they're pushing against a relatively immovable object — voters who mostly decided what they think of the president long ago.

When it comes to Romney, though, it's another story. Many voters still don't know much about the former Massachusetts governor except that he's a Republican, he's wealthy and he's a Mormon (and only about 60% know even that much).

That's why the Obama campaign and its allies have spent millions over the last month to try to define Romney in voters' minds — as a financier who shipped jobs overseas and stashed his money in the Cayman Islands — before he can introduce himself in more flattering terms.

And the results are measurable. This month, Rivers and John G. Geer of Vanderbilt University teamed up to launch an intriguing experiment: They're showing campaign advertisements to voters over the Internet, and then polling them to see whether they can detect an effect.

When Rivers and Geer showed voters Obama's toughest ad — the one that uses Romney's off-key rendition of "America the Beautiful" behind charges that Romney has sent jobs offshore and stashed money in overseas accounts — the results were dramatic.

Among independents who hadn't seen the commercial, Obama's lead was a bare 3%. But among independent voters who saw that commercial, Obama's lead over Romney swelled to 16%.

"That ad didn't offend independents," Geer said. "It moved independents from Romney toward Obama." Which is exactly what it was designed to do.

Overall, Rivers and Geer found, positive ads from either side didn't have a measurably significant effect on voters. Negative ads about Obama didn't have much of an effect either. The only commercials that seemed to have a clear effect were negative ads about Romney.

Still, the candidates are also airing a few positive ads touting their achievements or rebutting negative ads — but very few so far. And the negative ads won't come down soon, because as Geer and others have noted, the messages of political ads wear off quickly unless the campaigns hammer away at them, week in and week out.

As Obama once said during his bare-knuckled primary battle with Hillary Rodham Clinton: "I'm from Chicago…. We don't play in Chicago. We take our politics seriously."

The Obama onslaught has prompted plenty of hand-wringing in Republican ranks. Some GOP strategists have publicly questioned why the Romney campaign has failed to air more positive information about the candidate — more of his impressive biography, more commercials showing his photogenic family, more details about his economic plans.

Romney says he thinks he still has plenty of time for that. "This is still early for a lot of people in the political process," he told NBC News last week. "Labor Day is usually the time when people focus more attention on candidates…. Most folks won't really get to see me until the debates."

But the first debate won't come until Oct. 3, more than two months from now. Romney doesn't really mean to wait that long; the Republican convention at the end of August will be one long Romneyfest. Still, every week he delays in revealing more of his himself to the electorate, the fewer undecided voters may be left to listen.